Sunday, June 29, 2008

Combined Solar and Volcanic Aerosol Effect – Part 1

To attempt to properly investigate the possible solar origin of the late 19th to early 20th century plunge in SST, total solar irradiance (TSI) cannot stand alone. The impact of volcanic aerosols must be included. I attempted to find a data set that combined the two and uncovered multiple discrepancies in the GISS radiative forcing data that render that compilation useless to me.

GISS DATA

The following quote is from the GISS Surface Temperature Analysis, Global Temperature Trends: 2007 Summation: “This cyclic solar variability yields a climate forcing change of about 0.3 W/m2 between solar maxima and solar minima. (Although solar irradiance of an area perpendicular to the solar beam is about 1366 W/m2, the absorption of solar energy averaged over day and night and the Earth's surface is about 240 W/m2.) Several analyses have extracted empirical global temperature variations of amplitude about 0.1°C associated with the 10-11 year solar cycle, a magnitude consistent with climate model simulations, but this signal is difficult to disentangle from other causes of global temperature change, including unforced chaotic fluctuations.”http://data.giss.nasa.gov/gistemp/2007/

On another webpage, GISS discusses radiative forcings used in their climate models:http://data.giss.nasa.gov/modelforce/And they list separate radiative forcing data for numerous variables, including solar irradiance and volcanic aerosols.http://data.giss.nasa.gov/modelforce/RadF.txtFigure 2 illustrates the GISS solar radiative forcing. The curve is that of the Lean et al (2000), including background. Their use of the Lean et al data is confirmed in the GISS discussion “Climate simulations for 1880–2003 with GISS modelE”.http://pubs.giss.nasa.gov/docs/2007/2007_Hansen_etal_3.pdfThe accuracy of this data set was questioned by the author within a year because, in a later paper, she felt it was incorrect, yet GISS continues to use it. The second thing that stands out is the maximum-to-minimum variation in the last two solar cycles. It’s approximately 0.14 watts/meter^2, or less than half that listed in the GISS 2007 Summation of Temperature Trends. Does GISS need the erroneous data set calibrated to less than half of the accepted values in order to give more weight to anthropogenic forcings when they attempt to duplicate the global temperature record with their climate models?http://i25.tinypic.com/142aadh.jpgFigure 2

Figure 3 shows the radiative forcing of stratospheric aerosols resulting from explosive volcanic eruptions. It is based on the GISS Sato Index of Mean Optical Thickness, which, from my investigations, appears to be the best volcanic dust veil index available. Unfortunately, the Sato Index data only goes back as far as 1850. In this illustration, GISS presents the data in terms of watts/meter^2 from 1880 to 2003.http://i30.tinypic.com/2uzxe0k.jpgFigure 3

The solar and volcanic aerosol data are illustrated together in Figure 4. The magnitude of stratospheric (volcanic) aerosol forcing dwarfs that of solar irradiance. Looking at the Mount Pinatubo eruption of 1992 and the last two solar cycles, the volcanic aerosol radiative forcing is 20 times that of the min-to-max radiative forcing for the last two solar cycles. GISS claims this min-to-max change in solar irradiance is responsible for a 0.1 deg C change in global temperature, as noted above. That would mean, based on the relative magnitude of solar irradiance to stratospheric aerosols, that global temperatures dropped 2.0 deg C in response to the Mount Pinatubo eruption. That didn’t happen. The maximum estimate of the annual drop in temperature from Mount Pinatubo is 0.5 deg C and the minimum is 0.2 deg C.http://i32.tinypic.com/99oq46.jpgFigure 4

It’s a shame that GISS has to manipulate data to fit their purposes. If I had the time, I would investigate the other forcings GISS provides in the data set above, but I don’t and I won’t. The GISS data is skewed and worthless for my uses. It may work in the GISS climate models, but it will not work in a comparison to historical temperature record data.

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